NASA Kepler Finds Earth-size Planet Candidates in Habitable Zone, and a Six Planet System
Big news from the NASA Kepler team as they release more Kepler satellite data, and announce a fascinating discovery (all emphases mine) - from the press release above:
NASA’s Kepler mission has discovered its first Earth-size planet candidates and its first candidates in the habitable zone, a region where liquid water could exist on a planet’s surface. Five of the potential planets are near Earth-size and orbit in the habitable zone of smaller, cooler stars than our sun.
Candidates require follow-up observations to verify they are actual planets. Kepler also found six confirmed planets orbiting a sun-like star, Kepler-11. This is the largest group of transiting planets orbiting a single star yet discovered outside our solar system.
The discoveries are part of several hundred new planet candidates identified in new Kepler mission science data, released on Tuesday, Feb. 1. The findings increase the number of planet candidates identified by Kepler to-date to 1,235. Of these, 68 are approximately Earth-size; 288 are super-Earth-size; 662 are Neptune-size; 165 are the size of Jupiter and 19 are larger than Jupiter. Of the 54 new planet candidates found in the habitable zone, five are near Earth-sized. The remaining 49 habitable zone candidates range from super-Earth size — up to twice the size of Earth — to larger than Jupiter.
The findings are based on the results of observations conducted May 12 to Sept. 17, 2009, of more than 156,000 stars in Kepler’s field of view, which covers approximately 1/400 of the sky.
“The fact that we’ve found so many planet candidates in such a tiny fraction of the sky suggests there are countless planets orbiting sun-like stars in our galaxy,” said William Borucki of NASA’s Ames Research Center in Moffett Field, Calif., the mission’s science principal investigator. “We went from zero to 68 Earth-sized planet candidates and zero to 54 candidates in the habitable zone, some of which could have moons with liquid water.”
As I’ve emphasized, “candidates” still need confirmation, as false-positive signals are to be expected.
Nature magazine, which published the paper (behind pay-wall) about the confirmation of the 6 planet solar system has several survey articles (which are fortunately open access) in the same issue, including this short article looking at the history of the Kepler satellite and its discoveries and the importance of confirmation of candidates: Astronomy: Beyond the stars and an article on possible future techniques of planet detection and their costs: Astronomy: Exoplanets on the cheap
It also should be emphasized that all the data released are from only 5 months of Kepler observations (up through middle of Sept 2009). As data collected since then is analyzed, longer period planets (which are farther from their star) will show up on the candidate list.
Regarding the solar system (now labeled “Kepler 11”) announced which has 5 close in planets, a press release from UC-Santa Cruz adds:
[…] The five inner planets in the Kepler-11 system range in size from 2.3 to 13.5 times the mass of the Earth. Their orbital periods are all less than 50 days, so they orbit within a region that would fit inside the orbit of Mercury in our solar system. The sixth planet is larger and farther out, with an orbital period of 118 days and an undetermined mass.
“Of the six planets, the most massive are potentially like Neptune and Uranus, but the three lowest mass planets are unlike anything we have in our solar system,” said Jonathan Fortney, assistant professor of astronomy and astrophysics at UCSC, who led the work on understanding the structure and composition of the planets, along with UCSC graduate students Eric Lopez and Neil Miller.
“The timing of the transits is not perfectly periodic, and that is the signature of the planets gravitationally interacting,” he said. “By developing a model of the orbital dynamics, we worked out the masses of the planets and verified that the system can be stable on long time scales of millions of years.”
What an interesting solar system - 5 large planets all in orbits closer than mercury! It also lends credence to the idea that some solar systems are very dynamic - that planets rearrange themselves over long time periods. Orbital mechanics becomes very difficult to solve once more than the 3 bodies are involved (the famous n-body problem from physics). Thus the announcement about Kepler-11 indicates that the 5 close-in planets may only stay in their orbits for a few million years. Eventually one or more may fall into the star, or be ejected into an outer orbit. Astronomers now believe that in our own solar system Saturn and Neptune and Uranus made some large moves early on in the development of our system.
The Kepler-11 system has some more interesting traits (from the USSC presser):
As is the case in our solar system, all of the Kepler-11 planets orbit in more or less the same plane. This finding reinforces the idea that planets form in flattened disks of gas and dust spinning around a star, and the disk pattern is conserved after the planets have formed, Fabrycky said. “The coplanar orbits in our solar system inspired this theory in the first place, and now we have another good example. But that and the Sun-like star are the only parts of Kepler-11 that are like the solar system,” he said.
The densities of the planets (derived from mass and radius) provide clues to their compositions. All six planets have densities lower than Earth’s. “It looks like the inner two could be mostly water, with possibly a thin skin of hydrogen-helium gas on top, like mini-Neptunes,” Fortney said. “The ones farther out have densities less than water, which seems to indicate significant hydrogen-helium atmospheres.”
That’s surprising, because a small, hot planet should have a hard time holding onto a lightweight atmosphere. “These planets are pretty hot because of their close orbits, and the hotter it is the more gravity you need to keep the atmosphere,” Fortney said. “My students and I are still working on this, but our thoughts are that all these planets probably started with more massive hydrogen-helium atmospheres, and we see the remnants of those atmospheres on the ones farther out. The ones closer in have probably lost most of it.”
The inner planets are so close together that it seems unlikely they formed where they are now, he added. “At least some must have formed farther out and migrated inward. If a planet is embedded in a disk of gas, the drag on it leads to the planet spiralling inward over time. So formation and migration had to happen early on.”
As we continue to discover more and more solar systems one thing is becoming evident - planets and solar systems come in quiet the variety! We shouldn’t expect other planets to have to be too similar to any like the 8 in our own solar system.
The National Geographic website article on this discovery has an illustrative graphic to compare the Kepler-11 planet sizes with others.
Another good overview of today’s announcements can be found in the NYT article Kepler Planet Hunter Finds 1,200 Possibilities.
We are living in the golden age of astronomy. However, it really must be noted that the current proposed cuts in the NASA budget from the Tea Partying crowd would eliminate work such as done by the NASA Kepler satellite, which cost about 500 million dollars to build and launch, and requires continuing expenditure of several million dollars a year in data collection and analysis. A choice our society must make is whether this type of research, at a cost of a couple of dollars per citizen, will continue.